Semiconductor lasers

The term laser stands for Light Amplification by Stimulated Emission of Radiation. In a semiconductor laser a p-n junction can be used to obtain the population inversion. When trivalent impurity atoms are added in a pure semiconductor we get p-type semiconductor. In a p-type semiconductor, majority current carriers are holes and minority current carriers are electrons. Similarly we can get n-type semiconductor by adding pentavalent impurity atoms in a pure semiconductor.

In n-type semiconductor, majority current carriers are electrons and holes are the minority current carriers. When n-type and p-type semiconductors make contact with each other, a p-n junction is formed and the boundary connecting both the semiconductors is known as junction. Now, the majority holes in p-type semiconductor diffuse through the junction into n-type semiconductor and majority electrons in n-type semiconductor diffuse through the junction into p-type semiconductor. Thus an electric field is setup across the junction which opposes the diffusion of electrons and holes through the junction and finally stops it. A forward biased p-n junction is enclosed between two plane mirrors. One of the mirror is partially transparent and the other is heavily silvered. Now, the holes from p-region and electrons from n-region flow into the junction. These electrons and holes recombine in the depletion region and hence energy in the form of radiation is emitted. This radiation stimulates electrons from the conduction band of p-n junction to the valence band and hence laser beam is obtained.

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